I'll give you the answers I'm good at these.
21 = 20
21 explained
twice a number is 60 more than so take 50+50=100
100-60=40
40\20=2
(but use 20)
20 X 5 = 100
This is a GEOMETRIC sequence as each number is being multiplied by 1/4
The answer is 11.
-3+y=8
y= 8+3
y=11
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Okay.
-3x+y=8
y=8+3x or y=3x+8
Or x = (y-8)/3